On marine sands of the Clovelly-Noordhoek sand corridor, over and around TMS sandstone outcrops of Dassenberg Koppies. Adamson and Salter describe this species (they name it Cassine maritima) as "frequent on sand dunes from Muizenberg to Chapmans Bay southwards". If I haven't mucked up the ID, it's not something I see often enough around here to still consider frequent.